The present invention relates to an output split type hydrostatic transmission for a wheeled vehicle, industrial equipment and the like, and more particularly to an improved hydro-mechanical transmission which includes a hydraulically controlled speed differential means drivingly connected with an output means for transmitting the rotation torque of an input means to the output means and a hydraulic positive displacement pump-motor driven by the input means to hydraulically control a reaction element provided within the speed differential means in response to the displacement ratio thereof.